1. Field of the Invention
The present invention is in the field of ventilation and more specifically relates to a system through which the supply of outside air to a closed dwelling space or class room is controlled to maintain the freshness and hygiene of the air inside independent of any permanent air conditioning system that might or might not be present.
2. The Prior Art
Throughout man's history there has never existed a quantitative yardstick available to him for assessing the quality of indoor air. The common variables used to characterize indoor air are the temperature and the relative humidity (RH). While these variables are certainly important in deciding the perceived comfort of one's surroundings, they nonetheless bear little relevance in determining the actual quality of indoor air.
Good air quality is often described as being "fresh," "nice" or "invigorating." Similarly, terms such as "foul," "stale" or "stuffy" are frequently used to designate bad quality or unbearable air but without any quantitative definition of what is meant.
For as long as one can remember, air conditioning systems have been devised to provide "good" air inside buildings based upon the parameters of air temperature and RH.
The inability of air temperature and relative humidity to characterize air quality can best be exemplified by remembering those instances when one has walked into an air-conditioned room and experienced the unmistakable bad odor of stuffy air. The occurrence of this episode, especially in midsummer, is invariably caused by the air-conditioning system recirculating only indoor air in order to save energy. A similar situation prevails in the winter when a heating system recirculates only indoor air in order to avoid the needed heating of the much colder, but fresher, outside air.
Good quality air necessarily means that the air has adequate oxygen content, low carbon dioxide (CO.sub.2) concentration, and few dust particle and microorganism counts.
Among the first attempts to control the air quality of a closed space using additional parameters other than temperature and relative humidity was the work of Anderten, et al. in U.S. Pat. No. 4,164,172. They describe method and apparatus for controlling the amount of outside air introduced into a given space being ventilated as a function of the oxygen content of the air in the space in such a way as to minimize the energy expended in heating or cooling the air.
In U.S. Pat. No. 4,567,939 Dumbeck further disclosed the use of a general purpose programmed computer to control an air circulation type air-conditioning system as the operating equipment between temperature, RH and oxygen sensors, and control devices for modifying temperature, RH and oxygen content of the circulating air.
As alluded to briefly above, an adequate supply of oxygen is not the only criterion determining the quality of indoor air. The CO.sub.2 concentration level and the dust particle and microorganism counts are equally important in characterizing good air quality. While dust particles and microorganisms can usually be controlled by the use of appropriate filters and/or electrostatic precipitators, the control of carbon dioxide has remained elusive. It is by far the most significant indoor air pollutant.
Human beings and animals alike produce a large amount of CO.sub.2 by virtue of their body metabolism, namely the derivation of energy by the burning of carbohydrates in the presence of oxygen. The end products are water and carbon dioxide. Surprisingly, the level of CO.sub.2 at the end of each of our breaths reaches as high as 50,000 parts per million (ppm) for normal individuals (100% CO.sub.2 =1 million ppm of CO.sub.2).
While CO.sub.2 concentration can be as low as 300-400 ppm for pristine outdoor air, the indoor CO.sub.2 level averages between 600 to 1,500 ppm depending on how many people are present in a particular space and how airtight it is.
Although a high level of CO.sub.2 indoors by itself is not necessarily a health hazard provided that the level does not rise above 3,000-5,000 ppm for prolonged periods, a high level of CO.sub.2 (above 1,000 ppm) does indicate poor ventilation which leads to other problems as will be explained below.
The CO.sub.2 level is an extremely good parameter for indexing the degree of ventilation indoors. When ventilation is poor, the level of CO.sub.2 tends to remain high or even rises slowly. Conversely, since the CO.sub.2 level is invariably lower outdoors (typically 400-500 ppm) good indoor ventilation leads to much lower CO.sub.2 levels. This explains why CO.sub.2 level is such a good parameter for indexing indoor ventilation.
Coblentz was the first to recognize in U.S. Pat. No. 2,984,082 that comfort is affected by the freshness of the air as well as its temperature and humidity. He disclosed a method of using a CO.sub.2 meter to control the admission of outside air in an integrated air-conditioning system. The CO.sub.2 meter, as part of the overall air-conditioning system and positioned in the return air duct, actually measures the quantity of CO.sub.2 in the return air which is then used to open or close a motor-driven damper for controlling the intake of outside air. Nishimuro in Japanese Patent No. 0136928 disclosed a very similar air-conditioning system.
In U.S. Pat. No. 3,593,711, Staub and McGoff disclosed a dual canister chemical type life support system which can be used to remove excess CO.sub.2 from air continuously and periodically add oxygen to it as needed.
In another U.S. Pat. No. 4,631,872, Daroga disclosed the design of a nuclear blast and fall-out shelter which can accommodate a number of people completely enclosed for several weeks. The shelter is provided with an oxygen supply, an air conditioner, and a hand-operated pump for introducing external air to maintain adequate oxygen to sustain life.
Finally, Tokunaga in Japanese Patents Nos. 281549 and 281550 disclosed how an air-conditioning system can be augmented by a personnel sensor and a CO.sub.2 sensor in a room to effect the intake of outside air for maintaining the CO.sub.2 concentration of the room at a desired level.
Despite the numerous disclosures of methods and apparatus cited above and despite the correct recognition that adequate ventilation is the only effective way of ensuring good indoor air quality, the use of indoor CO.sub.2 level as an adjunct to temperature and humidity for optimizing the performance of heating/cooling systems is seldom practiced today. Instead, modern air-conditioning systems simply deliver adequate airflow to office buildings based upon body-counts, time of day and season. The concentration of carbon dioxide is generally not measured.
The reason why carbon dioxide level is generally not used to control the air-conditioning system can be traced back to the lack of a reliable, accurate and drift-free CO.sub.2 sensor. If the CO.sub.2 sensor drifts excessively or becomes erratic, the whole air-conditioning system will malfunction leading to an unacceptable situation.
Because of this lack of an accurate, reliable and drift-free CO.sub.2 sensor, most of the ordinary residences and small offices in this country continue to suffer from poor ventilation due to uncontrolled and often elevated CO.sub.2 levels indoors. The situation is particularly acute during the cold winter months when all doors and windows are shut tight in order to save heating fuel. A simple yet affordable ventilation controller which can work independently of the often cumbersome heating/cooling systems for homes or office buildings is clearly needed in order to alleviate indoor air pollution problems that result from poor ventilation.